Shipping bag

ABSTRACT

A thermoplastic heavy duty shipping bag having walls formed of cross-laminated outer plies of uni-axially oriented polyethylene and inner walls of low density polyethylene, heat sealed thereto. The bag provides improved puncture and snag resistance.

This is a continuation, of application Ser. No. 631,540, filed July 16,1984, now U.S. Pat. No. 4,576,844.

BACKGROUND OF THE INVENTION

This invention relates to thermoplastic shipping bags and, particularly,to said bags for heavy duty use having one or more plies of auni-axially oriented cross-laminated polyethylene.

Thermoplastic shipping bags are an economical means for the packaging,transportation and storage of a wide variety of products in granular,bead, pellet or powder form. These bags may be either open ended,requiring separate provision for closing, or fitted with a valvedopening. The most commonly used type of thermoplastic bags aremanufactured from film made by blown film processes using low densitypolyethylene and rubber modified high density polyethylene resins andcoextruded versions of these resins and allied copolymers. The film maybe subsequently converted to bags by a variety of sealing techniques.Some of these simple thermoplastic films may be converted to bags bysewing but this has the disadvantage of badly weakening the films andintroducing unwanted holes in the bag through which moisture or othercontaminants can enter or allow environmental contaminants to exit.Other bags may be manufactured by the multiple folding and glueing ofthe bag ends, but this has the serious disadvantage of requiring veryexpensive converting equipment, as well as the use of expensive anddifficult to control two component glue systems.

Because of these drawbacks the use of heat sealing is the desired methodof manufacture of thermoplastic bags.

At the present, heat sealed thermoplastic bags are commonly used for thepackaging of low cost, non-hazardous products. Their use for higher costand hazardous products, however, has been badly restricted because ofthe poor puncture and snag resistance of the bags compared withmulti-wall paper or woven plastic bags of related compositionstructures.

It is known in the art that the required level of puncture and snagresistance can be built into a thermoplastic film manufactured fromlinear ethylene polymers such as high density polyethylene or linear lowdensity polyethylene, by uni-axially cold drawing such film andsubsequently laminating two of these layers in such a manner that thedraw-induced orientation of one web typically is 90° to that of theother web. Thus, the uni-axially oriented sheets of polyethylene arelaminated one to the other in such a manner that the directions oforientation cross each other. The resulting composite laminated film,generally known as a cross-plastic, cross-laminate, or cross-laminateply has high puncture and snag resistance compared to an equivalentgauge non-oriented blown film. Films of this type have thus been usedfor sewn and glued shipping bags for high value and hazardous materials.

However, these materials have one serious drawback in that they cannotreliably be heat sealed on conventional bag making machinery to providebags for heavy duty use. This has seriously impeded their utility andacceptability in the market place notwithstanding the aforementionedadvantages of heat sealed bags.

SUMMARY OF THE INVENTION

Surprisingly, we have now found that this unacceptable heat sealdrawback can be overcome to produce an improved heavy duty shipping bagby interposing a double layer of low density polyethylene, which has notbeen cold drawn (i.e., has not been uni-axially oriented), between thetwo cross-laminated films to be heat sealed together to form the wallsof the bag. We have surprisingly found that these layers of low densitypolyethylene in the heat seal area can be welded to each other and tothe cross-laminated film or ply without heat build up sufficient tocause serious loss of cold draw-induced film strength. Thus, anacceptable bridge between the high strength cross-laminated film and thebody of the heat seal is formed. This is to be contrasted with the factthat although two cross-laminated films in the absence of interposed lowdensity polyethylene film could be melted and fused together to producewelded bonds, the cross-laminated film immediately adjacent to thewelded mass has its cold draw orientation destroyed by the heat from theseal with consequent loss of film strength in this margin area; wherebythe seals so produced are weak and brittle in the margin area, renderingthem unacceptable for use in heavy duty shipping bags.

It has thus now been found that a suitable thermoplastic shipping baghaving improved puncture and snag resistance can now be reliablymanufactured by heat sealing techniques using suitably modifiedconventional equipment.

Thus, in its simplest form the invention provides a thermoplasticshipping bag having a front wall and a back wall, each wall comprising across-laminated ply comprising at least two sheets of uni-axiallyoriented polyethylene bonded together; said laminated plies heat sealedone to the other to provide a heat seal area and wherein interposedbetween said laminated plies in said heat seal area are two layers oflow density polyethylene.

Each of the interposed layers of low density polyethylene may constitutesimply a sheet of polyethylene of sufficient area and thickness in theheat seal area to effect an acceptable bridge between the two laminatedplies in this area to form a seam. However, these interposed layers oflow density polyethylene may extend beyond the heat seal area torepresent a full ply adjacent the full surface of each of the laminatedplies. Thus, each of the cross-laminated plies comprising the walls ofthe shipping bag have a layer of low density polyethylene adjacentthereto. Such a structure, of course, does not detract from therequirement that the laminated plies need only be heat sealed atdesignated heat seal areas. These areas constitute those parts of thebag, generally parts of the periphery, where the front and back wallsare joined by heat sealing during manufacture. Each of the sheets of lowdensity polyethylene must be of sufficient thickness to effect anacceptable bridge between the two cross-laminated plies. We have foundthat a sheath of such thinness to provide mere coating of low densitypolyethylene on each of the cross-laminated plies is not sufficient, andthat a minimum thickness of 0.5 mil of low density polyethylene isrequired, preferably >1.5 mil.

We have also found that each of the cross-laminated plies constitutingthe walls of the bag must have its own associated sheet of low densitypolyethylene to provide an acceptable heat seal for heavy duty bag use.For reasons hereinafter discussed, a single interposed layer of lowdensity polyethylene, represented as a distinct ply, is notsatisfactory. Thus, a double layer of polyethylene is required.

In a much preferred form of a bag according to the invention theinterposed layers of low density polyethylene represent full anddistinct plies constituting part of the walls of the bag.

Accordingly, the invention further provides a bag as hereinbeforedescribed wherein each of said layers of low density polyethyleneconstitutes an inner ply of the bag.

In this preferred form of bag each of the walls comprising across-laminated ply has an interposing ply of low density polyethyleneassociated therewith. In this arrangement, each of the interposing pliesmay be considered as being an inner wall of the bag while the twocross-laminated plies considered as being the two outer walls.

Accordingly, the invention further provides a thermoplastic shipping bagcomprising a front wall and a back wall heat sealed thereto, each ofsaid walls comprising a cross-laminated outer ply comprising at leasttwo sheets of uni-axially oriented polyethylene bonded together, andheat sealed thereto, an inner wall formed of low density polyethylene.

When a bag containing a fluid product is dropped a major amount of thekinetic energy from the product is transmitted to the bag walls uponimpact as the walls prevent the product from flowing outward. Thisenergy is absorbed by the bag walls stretching. The peak force build-upin the bag walls during this energy absorption is dependent on theelasticity of the bag walls. If the peak force build-up on impact isgreater than the tensile yield strength of the bag walls the bag will bepermanently stretched. When this peak force is greater than the ultimatetensile strength of the walls the bag will rupture at the weakestpoints. When a heat seal is introduced into a bag walls the ultimatetensile strength of the heat seal at its weakest section should begreater than the tensile yield strength of the bag walls to maximize theimpact strength of the heat seal.

In uni-axially oriented cross-laminated cold drawn films, such as"VALERON"* high density polyethylene, the yield tensile strength of thefilm is higher than the ultimate tensile strength of the plastic resinmass from which it is made. When the film is heat sealed and orientationof the film is destroyed in the heat seal margin which results in adrastic reduction in both the yield tensile strength and the ultimatetensile strength.

A `laminated seal` is understood in the art to be a seal wherein thejoining interfaces of the films can be separated by the physical pullingapart of the bonded film without the destruction of the film and thuseffecting a relatively clean separation. On the other hand a `heat seal`in the art is understood to be a seal produced under a combination ofpressure and heat, at or above the films' crystalline melting points,applied to the films in order that they are truly welded at theirinterfaces such that a clean separation cannot be effected by physicalor chemical means.

We have surprisingly found that a heat seal as hereinbefore defined whenintegrally formed with an adjacent laminated seal has improved sealstrength. Thus, by the term `heat seal` as used in this specificationand claims is meant a heat seal as hereinbefore defined having anadjacent laminated seal integral therewith. The adjacent laminated sealhas a length of at least 2 mm.

We have found that in order to prevent stress from building up only inthe heat sealed junction of the oriented and non-oriented films theselayers must be laminated sealed together adjacent the heat seal mass sothat the stress is also taken up by the adjacent multi-ply laminate. Itwill be realized that film stretching is a two dimensional effect, i.e.if a film is stretched only in one direction it tends to neck down inthe other if unrestrained. Therefore, if the inner non-oriented layer istotally laminated to the less elastic oriented outer layer it will takeon the neck down characteristics of the total laminate. It isnonetheless necessary that laminations extend between the layers inclose proximity to the heat seal. A length of only 5-10 mm for theadjacent laminated seal integral with the heat seal mass is the mostdesirable length. Further, we have found that a single layer ofnon-oriented polyethylene between the two oriented layers reduces theutility of this invention since that laminated section of the sealbetween the single ply and the double ply peels away under the constantpressure exerted when bags are loaded and stacked.

By the term "thermoplastic", as used herein is meant any thermoplasticsmaterial capable of providing a film, ply or layer of suitable thicknessand strength for a heavy duty shipping bag. Of particular use arethermoplastics of the polyethylene and polybutadiene family of polymers.As examples, high density and low density polyethylenes and 1,2polybutadienes may be mentioned.

The term "low density polyethylene" includes ethylene homopolymers andcopolymers, such as the linear low density polyethylenes, vinyl acetatecopolymers, and blends thereof.

The term "inner wall" is meant not to be restricted solely to the actualof true inner wall of the bag which contacts product when the bag isfilled. The term also includes the situation, for example, where one ormore plies of non-oriented low density polyethylene constitute plies ina multi-wall bag which plies may or may not be adjacent the true innerwall. Similarly, the term "outer ply" is meant not to be restrictedsolely to the most external ply.

Thus, it should be understood that the principles of the invention areapplicable also to the fabrication of bags having walls individuallycomprising more than two plies. Thus, the invention embraces bags havingthree plies, four plies, etc. The important and essential feature isthat there must be at least one ply of non-oriented low densitypolyethylene constituting each of the inner surfaces of the bag suchthat a cross-laminated ply of polyethylene does not contact anothercross-laminated ply of polyethylene at a designated heat seal area of aninner surface such as to weaken a heat seal.

In preferred embodiments of the bags according to the invention ashereinbefore and hereinafter defined the interposed layer of low densitypolyethylene represented as a distinct inner ply or inner wall, isformed of blown linear low density polyethylene. However, it is readilyapparent that cast films are also suitable for this application.

Preferably each of the cross-laminated plies comprises at least twosheets of uni-axially oriented low density polyethylene, more preferablylinear low density polyethylene, and yet, more preferably, high densitypolyethylene.

Where the cross-laminated ply comprises sheets of uni-axially orientedhigh density polyethylene, the sheets may be bonded together, forexample, with a layer of non-oriented high density polyethylene. Wherethe laminated ply comprises sheets of uni-axially oriented low, orpreferably, linear low density polyethylene, these may be bondedtogether with non-oriented low density polyethylene or, preferably,linear low density polyethylene.

In one form, bags according to the invention comprise two inner liningsor walls of non-oriented low density polyethylene film, heat sealed attheir peripheries to plies of cross-laminated polyethylene film. The lowdensity polyethylene inner lining or wall may, however, be also heatsealed to the cross-laminated ply intermittent over other parts of theirrespective facing surfaces, provided that there is not sufficient heatbuild-up to cause serious loss of film strength in the cross-laminatedply. Generally, the inner wall and cross-laminated outer ply are heatsealed at and around all, or part of, their peripheries; these being theprincipal heat seal areas. The arrangement and positions of the heatseal areas will depend on the type of shipping bag, and it is wellwithin the skill of the art for such areas to be identified.

While it is generally accepted that all polyethylene film is generallyuni-axially oriented to some degree, the term "uni-axially oriented"when used with reference to polyethylene in this specification andclaims means polyethylene film that has been blown and cold drawn to atleast a 2.5-fold extent, preferably to a 4-fold extent, but also up to a6-fold extent. The orienting and cross-laminating of the films may becarried out according to well-known methods.

A typical uni-axially oriented cross-laminated ply may be made byextruding respective tubular high density polyethylene or low densitypolyethylene film and cold drawing this tubing down by a factor of fourtimes in the machine direction to produce film with extremely high MDtensile and TD tear properties. The tubing is then subsequently spirallysplit into sheeting whereby the film orientation is typically at anangle of 45° to the new web MD machine direction. Two sheets of thissheeting may then be extrusion laminated with a thin layer ofnon-oriented high density polyethylene or low density polyethylene as isappropriate.

The cold drawn cross-laminated film may be made from low density andhigh density polyethylene resins and blends thereof, and can be used ina variety of thicknesses. One particular blend of use in the practice ofthe invention comprises linear low density and high densitypolyethylenes in the ratio of 9:1. Increasing the relative amount ofhigh density polyethylene in such a blend enhances the punctureresistance and tensile strength of the film.

It is not necessary that all plies of the bag be made of the samematerials as those of use in the practice of the invention. With theability to introduce compatible but dissimilar materials, speciallydesigned bags can be produced with plies allowing the packaging of finepowders, oil-bearing materials, materials that are hot at the point ofpackaging, etc.

Thus, the bags according to the invention may also comprise one or moreplies formed of other thermoplastic compatible packaging materialswithout detracting from the principles of the invention.

Also, it will be understood that the scope of the invention as claimedalso embraces those thermoplastic bags wherein only a part of themanufactured bag incorporates the important and essential feature of theinvention as hereinbefore set forth and wherein other parts of the bagare joined in an alternative manner, for example, by hot melt oradhesive bonding. The arrangement and positions of those other partsjoined in an alternative manner may be readily selected by the skilledman.

A two-ply bag is the simplest embodiment of this invention. However, insome instances it is advantageous to have more than two inner plies ofnon-oriented film constituting the inner layers of the bag, i.e.,between the front and back cross-laminated outer sides of the bag. Anexample of this would be a bag of the simplest embodiment with anadditional thin true inner ply of linear low density polyethylene in theform of a fine filter mesh to allow air to be filtered from powderedproducts, as described in our copending Canadian Application Ser. No.438,484, filed Oct. 6, 1983.

In other instances it may be preferred to have additional plies of filmoutermost of the cross-laminated ply. Such an outer ply could give thebenefit resulting from introducing blown low density polyethylene filmbetween the gussetted surfaces of cross-laminated plies to give the sameimprovements in seal quality as created on the innermost parts of thebag. The squared-off appearance of the final package resulting from thisgussetting improves its performance for palletizing and stacking.

An additional benefit to be gained from such an outer layer is that thesurface can be suitably roughened by the addition of high molecularweight granules to the film during film extrusion; thus, impartingadditional improved handling properties to the bag. As well, the innersurface of this outer ply can be printed and the resulting message thuslocked between plies to escape abrasion and distortion during thehandling of filled packages. It can readily be seen that the utility ofthis outer ply can be expanded by using a laminate or coextrusion filmto impart special properties to the bag, i.e., oil barrier or greaseresistant layers.

Accordingly, in a further feature the invention provides a thermoplasticshipping bag of the open-top type having a front side and a back sidecharacterised in that each of said front side and said back sidecomprises a cross-laminated outer ply comprising at least two sheets ofuni-axially oriented polyethylene bonded together, and heat sealedthereto, an inner wall formed of low density polyethylene.

An open-top shipping bag according to the invention may be made byfeeding a web of the uni-axially oriented crosslaminated film inconjunction with an inner web of blown low density polyethylene throughcommercial side-weld, heat sealed or back seamed and bottom heat sealedbag making equipment.

One particularly useful type of thermoplastics shipping bag is thatknown as a valved bag. One such embodiment is described in our U.S. Pat.No. 3,833,166. These bags posses the important commercial advantage ofbeing easily filled through a valve structure with the self-closing ofthis valve structure after filling.

Accordingly, in a further feature the invention provides a thermoplasticshipping bag of the valved bag type having a front side and a back sidejoined together around the entire periphery of the bag and a fillingaperture characterised in that each of said front side and said backside comprises a cross-laminated outer ply comprising at least twosheets of uni-axially oriented polyethylene bonded together, and heatsealed thereto, an inner wall formed of low density polyethylene.

In a more preferred feature the invention provides a thermoplasticvalved bag of the type having a front side and a back side joinedtogether around the entire periphery of the bag, said front sideconsisting of a first panel and a second panel, of greater combinedwidth than the width of said back side, said first panel at leastpartially overlapping said second panel throughout the length of the bagand said panels in their common area being joined together along a linesubstantially parallel with and at a distance from one end of the bag,thus forming a tubular self-closing filling sleeve having inner andouter walls and extending transversely of the bag adjacent to said oneend thereof, with said first panel forming the outer wall and saidsecond panel forming the inner wall of said filling sleeve, and beingalso joined together along at least one line extending from saidfirst-mentioned line substantially to the opposite end of the bag, saidsecond panel consisting of at least two plies that are non-coextensivewith each other so that at least the inner end portion of the inner wallof said filling sleeve is formed of a number of plies that is less thanthe total number of plies in said second panel, characterised in thateach of said front side and said back side comprises a cross-laminatedouter ply comprising at least two sheets of uni-axially orientedpolyethylene bonded together, and heat sealed thereto, an inner wallformed of low density polyethylene.

Several embodiments of this invention will now be more particularlydescribed by way of example only with reference to the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view of an open-top bag according to theinvention;

FIG. 2 is a sectional view along line II--II of FIG. 1;

FIG. 3 is a front elevational view of a valved bag according to theinvention;

FIG. 4 is a sectional view along the line IV--IV of FIG. 3;

FIG. 5 is a diagramatic view of a section through a heat seal ashereinbefore defined of use in the practice of the invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 and 2 show a generally rectangular two-ply pillow-type bag 1having an inner wall 2 formed of blown linear low density polyethylenefilm (3 mil) manufactured from "2045" linear low density polyethyleneresin (Dow Chemical Co.), and an outer ply 3 (3.5 mil) of uni-axiallyoriented cross-laminated linear low density polyethylene film,commercially available under the trade mark "VALERON"*.

The bag 1 has thus a two-ply back wall 4, and a two-ply front wall 5made up of first and second partially overlapping panels 6 and 7. Theouter ply 3 of back wall 4 is continuous with the outer wall 3 of frontwall 5 except where separated and joined together by heat sealing withlayer 2 in the overlapping panels 6 and 7. Thus, the walls 4 and 5 areintegral and form a two-ply tube. One end of the tube 8 is heat sealedto form a simple two-ply open-top bag.

The bag was made by feeding a web of 37" film 3 into a longitudinalfolding frame with a web of film 2 and forming a two-ply tube 18" widewith a 1" overlapping portion. The four plies of the overlapping areawere then heat sealed longitudinally to consolidate the two-ply tubingwhich was then passed to a transverse heat seal unit which made thebottom seal 8. A 26" length of tube with the heat seal present was cutfrom the web by a guillotine to form the open top bag 1.

To test the strength of the heat seals, the bag 1 was filled with 50pounds of granular salt, heat sealed at its open end by a "Dough boy"heat sealer, and drop tested on each side, edge and butt from a heightof 10 feet. There was no rupture of any film or seal. The open top ofthe bag is generally heat sealed after filling with product to producean airtight and watertight package. Because it is extremely difficult toexclude all air from the filled package prior to the heat sealingoperation, it is preferable to perforate the walls of the bags withpinholes typically 0.025" in diameter to facilitate air release, thenumber of holes required depending on the amount of air left in the bagand the type of product being packaged. In those cases where it iscritical that the package retains its maximum value for air tightnessand moisture protection, the perforation holes in the inner and outerplies are offset typically by 11/2" to create an indirect path toair/product mixes during the venting period.

Although the inner ply 2 of the bag is described as a single ply ofsheeting it can be readily appreciated that a two-ply tube of 1.5 milcould also be used instead. Indeed since tubing may be less expensive tomanufacture the tube could be a preferred option. Again, although theouter wall is described as a linear low density polyethylenecrosslaminated film, high density polyethylene cross-laminated filmwould be the preferred embodiment for applications where extra heatresistance is required of the package.

FIGS. 3 and 4 show a generally rectangular threeply pillow type bag 10having a front side 11 and a back side 12 joined together around theentire periphery of the bag. Front side 11 consists of an inner wall 13and an outer wall 14 formed of blown linear low density polyethylene (3mil), and a middle wall 15 of uni-axially oriented cross-laminatedlinear low density polyethylene film, (3.5 mil) "2045" (Dow ChemicalCo.) resin. Back side 12 is of an identical construction.

Front side 11 has partially overlapping panels 16 and 17 heat sealedtogether longitudinally to form a three-ply tube open only to form aself-closing filling sleeve 18. The tube is heat sealed at both ends 19to form a complete valved bag of the type illustrated in our U.S. Pat.No. 3,833,166. In the embodiment shown the bag has its lateral edges 20tucked in and heat sealed in the longitudinal region 21 through twelvelayers of film.

It will be noted that in all the heat seal areas the uni-axiallyoriented cross-laminated film is never sealed to itself but always has adouble layer of non-oriented film between mating seal surfaces even inthe twelve-ply heat seal areas 19.

It is, of course, desirable to have this tucked-in multiple seal area togive the filled bag a squared configuration.

Again, it will be appreciated that tubing could be substituted forsheeting in layers 13 and 14.

Besides the advantage of allowing the bag to be gusset sealed, the outerply can be reverse printed to lock the print between plies 14 and 15 toprotect it from abrasion in transit. Additionally, a small amount (0.5%)of 40 mesh high molecular weight high density polyethylene resin can beincorporated in layer 14 during extrusion to produce a pebbled surfaceto confer excellent handling properties on the filled bags.

FIG. 5 shows a polyethylene heat seal in a bag loaded with productwherein the seal is under tension due to the product acting in a mannertending to separate the plies. The figure shows a polyethylene heat sealmass 50 resulting from the fusion of part of the two oriented highdensity polyethylene films 51 and the two non-oriented linear lowdensity polyethylene films 52. Integral with heat seal mass 50 at heatseal margin 53 are laminated seals 54, extending along each of the twoadjacent plies 51 and 52. There is a relatively minor lamination seal 55between the two plies 52.

The presence of the laminated seal integral with the heat seal can beaccomplished by the application of a gradient heat seal bar unit to thefilms whereby the lamination is effected at the same time as the heatseal. Alternatively, it can be accomplished in a two stage operationwherein a laminated seal is first made, typically of a 1" width, byjoining the plies at a temperature lower than the melting point of thecross-laminated polyethylene (to prevent destruction of theorientation), typically, 240° F. Subsequently, a side weld heat seal ismade through the laminated section by the application of temperature andpressure.

We have found that the thickness of the inner layers of low densitypolyethylene required to produce an acceptable heat seal will dependgreatly on the elasticity of the cross laminate film to be used, i.e.the less elastic the cross laminate film the thicker the low densitypolyethylene film must be. Relative thicknesses of all the polyethylenelayers can be readily determined by the skilled man.

We claim:
 1. A thermoplastic shipping bag comprising a front wall and aback wall heat sealed thereto, each of said walls comprising across-laminated outer ply comprising at least two sheets of uni-axiallyoriented polyethylene bonded together, and heat sealed thereto, an innerwall formed of low density polyethylene, said inner wall comprising adistinct inner ply.
 2. A bag as claimed in claim 1 wherein said innerwall includes linear low density polyethylene.
 3. A bag as claimed inclaim 1 or claim 2 wherein each of said cross-laminated outer pliescomprises at least two sheets of uni-axially oriented high densitypolyethylene bonded together.
 4. A bag as claimed in claim 1 or claim 2wherein each of said cross-laminated outer plies comprises at least twosheets of uni-axially oriented low density polyethylene.
 5. A bag asclaimed in claim 1 or claim 2 wherein each of said cross-laminated outerplies comprises at least two sheets of uni-axially oriented linear lowdensity polyethylene.
 6. A thermoplastic shipping bag of the open-toptype having a front side and a back side, each of said front side andsaid back side comprises a cross-laminated outer ply comprising at leasttwo sheets of uniaxially oriented polyethylene bonded together, and heatsealed thereto, an inner wall formed of low density polyethylene saidinner wall comprising a distinct ply.
 7. A bag as claimed in claim 6wherein said inner wall includes linear low density polyethylene.
 8. Abag as claimed in claim 6 or claim 7 wherein each of saidcross-laminated outer plies comprises at least two sheets of uni-axiallyoriented high density polyethylene bonded together.
 9. A bag as claimedin claim 6 or claim 7 wherein each of said cross-laminated outer pliescomprises at least two sheets of uni-axially oriented low densitypolyethylene.
 10. A bag as claimed in claim 6 or claim 7 wherein each ofsaid cross-laminated outer plies comprises at least two sheets ofuni-axially oriented linear low density polyethylene.
 11. Athermoplastic shipping bag of the valved bag type having a front sideand a back side joined together around the entire periphery of the bagand a filling aperture characterized in that each of said front side andsaid back side comprises a cross-laminated outer ply comprising at leasttwo sheets of uni-axially oriented polyethylene bonded together, andheat sealed thereto, an inner wall formed of low density polyethylene,said inner wall comprising a distinct ply.
 12. A bag as claimed in claim11 wherein said inner wall includes linear low density polyethylene. 13.A bag as claimed in claim 11 or claim 12 wherein each of saidcross-laminated outer plies comprises at least two sheets of uni-axiallyoriented high density polyethylene bonded together.
 14. A bag as claimedin claim 11 or claim 12 wherein each of said cross-laminated outer pliescomprises at least two sheets of uni-axially oriented low densitypolyethylene.
 15. A bag as claimed in claim 11 or claim 12 wherein eachof said cross-laminated outer plies comprises at least two sheets ofuni-axially oriented linear low density polyethylene.
 16. Athermoplastic shipping bag of the valved bag type having a front sideand a back side joined together around the entire periphery of the bag,said front side comprising a first panel and a second panel, said firstand second panels having a greater combined width than a width of saidback side, said first panel at least partially overlapping said secondpanel throughout a length of the bag, said panels in their overlappingarea being joined together along a line substantially parallel with andat a distance from one end of the bag, thus forming a tubularself-closing filling sleeve having inner and outer walls and extendingtransversely of the bag adjacent to said one end thereof, with saidfirst panel forming the outer wall and said second panel forming theinner wall of said filling sleeve, said second panel comprising at leasttwo plies that are non-coextensive with each other so that at least aninner end portion of the inner wall of said filling sleeve is formed ofa number of plies that is less than the total number of plies in saidsecond panel, each of said front side and said back side comprises acrosslaminated outer ply comprising at least two sheets of uniaxiallyoriented polyethylene bonded together, and heat sealed thereto, an innerwall formed of low density polyethylene said inner wall forming adistinct ply.
 17. A bag as claimed in claim 16 wherein said inner wallincludes linear low density polyethylene.
 18. A bag as claimed in claim16 or claim 17 wherein each of said cross-laminated outer pliescomprises at least two sheets of uni-axially oriented high densitypolyethylene bonded together.
 19. A bag as claimed in claim 16 or claim17 wherein each of said cross-laminated outer plies comprises at leasttwo sheets of uni-axially oriented low density polyethylene.
 20. A bagas claimed in claim 16 or claim 17 wherein each of said cross-laminatedouter plies comprises at least two sheets of uni-axially oriented linearlow density polyethylene.